A valence bond study of the Bergman cyclization: Geometric features, resonance energy, and nucleus-independent chemical shift (NICS) values
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The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1 enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space (CASSCF(6x6)} computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6x6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate art: non-synchronous; the TS is geometrically approximate to 80% product-like and electronically approximate to 70% reactant-like. While the pi resonance in the TS is very small, it is large (64.4 kcal mol(-1)) for 2 (cf. benzene = 61.5 kcal mol(-1)). As a consequence, substituents operating on the sigma electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional sigma resonance in 2 results in unusually high values for the nucleus-independent chemical shirt (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to sigma resonance to which the NICS procedure is relatively sensitive.
CitationCHEMISTRY-A EUROPEAN JOURNAL，2000,6（8）：1446-1454